The basic theory and operation of thermoelectric devices has been developed for many years. Thermoelectric devices are essentially small heat pumps which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other apparatus used to transfer heat energy. The principal difference is that thermoelectric devices function with solid state electrical components (thermocouples) as compared to more traditional mechanical/fluid heating and cooling components. Thermoelectric devices operate using the Peltier effect.
The circuit for a simple thermoelectric device generally includes two dissimilar materials such as N-type and P-type thermoelectric semiconductor elements. The thermoelectric elements are typically arranged in an alternating N-element and P-element configuration. In many thermoelectric devices, semiconductor materials with dissimilar characteristics are connected electrically in series and thermally in parallel. The Peltier effect occurs when voltage is applied to the N-type elements and the P-type elements resulting in current flow through the serial electrical connection and heat transfer across the N-type and P-type elements in the parallel thermal connection.
Modern thermoelectric systems typically include an array of thermocouples which operate by using the Peltier effect to transfer heat energy, an external power supply to convert AC power to DC power to produce the Peltier-type heat transfer with the thermocouple array, and external temperature control circuit. When electrical power is applied to a typical thermoelectric device having an array of thermocouples, heat is absorbed on the cold side of the thermocouples and passed through the thermocouples. A heat sink (sometimes referred to as the "hot sink") is preferably attached to the hot side of the thermoelectric device to aid in dissipating heat from the thermocouples to the adjacent environment. In a similar manner, a heat sink (sometimes referred to as a "cold sink") is often attached to the cold side of the thermoelectric device to aid in removing heat from the adjacent environment. Thermoelectric devices are sometimes referred to as thermoelectric coolers. However, since they are a type of heat pump, thermoelectric devices can function as either a cooler or heater.
In a typical thermocouple array, the direction of heat transfer is indicated by the direction of net current flow through the thermocouples. AC power does not generally affect heat transfer in a thermoelectric devices because AC power normally produces the same amount of current flow in alternating directions through the thermocouple array or essentially zero net current flow. Therefore, AC power, without modification or conditioning, applied to a thermocouple array results in no net transfer of heat energy. A typical thermoelectric device requires DC power in order to produce a net current flow through the thermocouples in one direction. The direction of the current flow determines the direction of heat transfer across the thermocouples. Therefore, the direction of net, non-zero current flow through the thermocouples determines the function of the thermoelectric device as either a cooler or heater.
Traditionally, there are various ways external components have been used to generate this DC power. One way is to drive the thermocouple array directly from a battery or external DC power supply. The external DC power supply often uses a bulky transformer to convert 120 volt or 240 volt AC power to DC power to drive the associated thermoelectric device. In many external DC power supplies, the AC line voltage is reduced using an external transformer. The transformed AC power is then rectified to DC power using an external rectifier circuit. This DC power is then applied to the thermoelectric device to produce the desired heat transfer.
Additionally, whenever it is desired to control the power supplied to the thermoelectric device, an external control circuit has traditionally been used to vary the current flow and resulting heat transfer rate. The external control circuit usually consists of a sensing element to sense the temperature and/or heat transfer rate, a feedback circuit to transfer this temperature or heat transfer information to the control circuit, and a regulator to adjust the net power flow to the thermoelectric device. Controlling the power flow controls the heat transfer rate. Often, the heat-transfer control circuit has used either proportional control, regulating the heat transfer rate by varying the amount of current to the thermoelectric device, or switching control, turning the current to the thermoelectric device off and on as necessary for the desired net heat transfer.